GTalk2VoIP is a site that promises to transform Google Talk into a full VoIP application, so you can call from Google Talk to a phone number or from a phone number to Google Talk. Another interesting thing you can do is to talk to a MSN Messenger user and even create a voice conference with users from Google Talk and MSN Messenger (Windows Live Messenger).
You don't need to install a software, you just have to add a contact (email@example.com) and use it as a gateway.
The creator of this service seems to have a lot of interesting ideas, but I hope it won't take too long before Google starts to offer similar features.
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